The present invention relates to an optical waveguide device fabricated by forming an acoustic surface wave light deflection or light modulation element on an optical waveguide substrate and a manufacturing method of surface acoustic wave element. In an integrated optics constituting a lens, optical switch, and other optical elements on an optical waveguide substrate, the various applications of an acoustic surface wave (SAW: Surface Acoustic Wave) light deflection element or SAW light modulation element using acousto-optic effect are in progress as a typical active element. Hereinafter, the SAW light deflection element and SAW light modulation element are collectively referred to as SAW element.
As to the above-mentioned SAW element, there have been discussions made public in "Integrated Optics" by Hiroshi Nishiyama et al published by Ohm Inc. in Japan (1983), pp. 118-126, pp. 320-335, pp. 361-369, and IEEE Proc. IGWO '89 (1989), pp. 138-141, for example, and it is possible to perform deflection of light, and modulation of intensity and frequency by affecting light traveling in the optical waveguide in the form of collinear or coplaner.
FIG. 7 is a view showing a covnentional SAW element. On a substrate 1 of lithium niobate or others, a thin optical waveguide layer 2 having a refractive index higher than the substrate is formed by ion exchange process or the like. Then, an incident laser light 9 is guided into this waveguide 2. Meanwhile, when a high-frequency alternating voltage 5 is applied to a comb-shaped SAW electrode 3 through a terminal 4, an surface acoustic wave (SAW) 6, the refractive index and displacement of the SAW element of which change periodically, is generated. If the incident laser light 9 is irradiated onto this SAW 6, the SAW 6 functions as a diffraction grating to change the frequency of the high-frequency alternating voltage 5; thus making it possible to perform the deflection, modulation, and the like for the emitting laser light 10. A reference numeral 7 in FIG. 7 designates an absorbent which terminates the propagation of the SAW 6. FIG. 8 is a view illustrating the state of propagation of SAW 6. The above-mentioned SAW is a wave 30 traveling on the surface of the optical waveguide. The crystal grating 32 on the optical waveguide element represents a locus of motion such as designated by a numeral 31 within the depth substantially equal to the wavelength .LAMBDA. of the traveling wave 30 at the time of SAW propagation.
In this respect, the wavelength .LAMBDA. of the traveling wave 30 is expressed by an equation (1) given below. EQU .LAMBDA.=v/f (1)
where f is the frequency of the high-frequency alternating voltage 5 applied to the SAW electrode 3 shown in FIG. 7, and v is the sound velocity on the optical waveguide 2.
In the above-mentioned conventional technique, there is a problem that no protection for the surface layer is considered in spite of the fact that the SAW is a phenomenon which appears on the surface layer of the optical waveguide.